Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream of the engine by using the flow of hot exhaust gas exiting the engine. The turbocharger typically includes a turbine housing with an inlet that receives exhaust gas exiting the engine such that the exhaust gas spins a turbine in the turbine housing. The turbine is mounted in the turbine housing on a shaft that is common to a radial air compressor housed in a compressor housing. Thus, rotary action of the turbine also causes the air compressor to spin within the compressor housing. The spinning action of the air compressor causes intake air to enter the compressor housing and to be pressurized or boosted a desired amount before it is mixed with fuel and combusted within a combustion chamber of the engine.
The turbine and compressor housings are typically mounted on first and second opposite sides of a center housing. The shaft extends between the turbine and the compressor through a bore in the center housing. An annular area of the first side of the center housing that extends around the shaft can be exposed to the inside of the turbine housing and, hence, the hot exhaust gas passing therethrough. The center housing also has a turbine mounting flange that extends radially outward from the first side of the center housing and is bolted to the turbine housing.
The center housing can define one or more lubricant passages for providing lubricant to the shaft and one or more coolant passages for circulating a coolant fluid such as water. The coolant passage can be an annular passage in the center housing that is proximate to the first side of the center housing and to the turbine housing. As a result of the difference in temperature between the hot exhaust gas in the turbine housing and the relatively cool lubricant and/or coolant fluid in the passages, high thermal gradients result in the center housing causing thermal stresses to develop. Thermal stress can also result from the temperature variations that occur over time during operation of the turbine. For example, the center housing is exposed to thermal transients, or variations, due to changes in the engine exhaust gas temperature over time that occur during normal engine operation. These thermal transients typically result in alternating cycles of heating and cooling of the center housing. During the heating cycles, the center housing can become hot enough to plastically deform, and stresses in the center housing that occur during the cooling cycles can be great enough to cause cracks to form. The likelihood of cracking or other stress damage is often greatest near features in the center housing, such as bolt holes that are provided in the mounting flange. Further, when liquid coolants such as water are circulated through the coolant passage, the higher cooling effect can result in even greater thermal gradients and greater or faster temperature variations in the center housing, thereby increasing the stress in the center housing and increasing the likelihood of cracking. Cracks that originate in the mounting flange can cause the housing to leak or otherwise fail.
Thus, there exists a need for an improved center housing that is characterized by reduced thermal stresses resulting from temperature variations during heating and cooling and from thermal gradients that exist between the hot exhaust gas and the relatively cooler lubricant and/or cooling fluid. The center housing design should reduce the likelihood of cracking or other failure of the center housing, for example, when cool liquid coolants are circulated through passages in close proximity to the first side of the center housing, which is exposed to the hot gas from the turbine housing.